Prologue to the Rule (paragraph 8)

If, however, you find in it anything which seems rather strict, but which is demanded reasonably for the correction of vice or the preservation of love, do not let that frighten you into fleeing from the way of salvation; it is a way which is bound to seem narrow to start with.

(From para. 8 of the Prologue to Saint Benedict's Rule, trans. by Patrick Barry OSB, 1997.)

In my experience, the mantra corrects the vice of distraction, and "preserves" selfless attention. I agree that it's a way which seems narrow to start with. But, through faithful practice, and in God's own time, I know it's the way for me to clarity and freedom.